Homework Help: Radius of electron trajectory

1. Nov 18, 2012

11thHeaven

1. The problem statement, all variables and given/known data

Electrons (mass m, charge –e) are accelerated from rest through a potential difference V and are then deflected by a magnetic field B that is perpendicular to their velocity. Find the radius of the resulting electron trajectory.

2. Relevant equations
$$F=\dfrac{mv^2}{r}$$ $$F=qvB$$

3. The attempt at a solution

I don't know where to start with this. The electron moves through the potential difference, and presumably gains kinetic energy, but I don't know how to express this in the equations above to get an answer.

Help appreciated.

2. Nov 18, 2012

grzz

I am assuming that the e is accelerated through a gap across which there is a potential difference V. What is the width of the gap?

3. Nov 18, 2012

11thHeaven

That's all the info I'm given; I've literally copied and pasted the question.

I'm not being asked to give a numerical answer though. It's a multiple choice question, and this is apparantly the answer:

$$\frac{\sqrt{2mV/e}}{B}$$

4. Nov 18, 2012

grzz

The p.d. gives an energy of Ve to the electron.

From this one can find the resulting velocity of the electron.

5. Nov 18, 2012

11thHeaven

Aha! Got it. Thanks for the help :)